In Cloud Firestore rules - How do I check if a key is null

Looking at the docs - https://firebase.google.com/docs/reference/rules/rules.Map

k in x  - Check if key k exists in map x

so this should work (without the keys())

!('assignee' in resource.data) 

if you want to make sure a key is null you need to check that this key is not part of the resource keys property:
!resource.data.keys().hasAny(['assignee'])

you can also use hasAll or hasOnly. more info here

Reading the list comparisons of the Firestore Security rules documentation here, we can see that hasAll returns true if all values are present in the list.

// Allow read if one list has all items in the other list
allow read: if ['username', 'age'].hasAll(['username', 'age']);

The request.resource.data is a map containing the fields and values. In order to use hasAll, we must first get the keys as a list of values as shown here.

!resource.data.keys().hasAll(['assignee'])